Method for producing biodiesel

ABSTRACT

A method for producing biodiesel includes feeding methanol into a tank and heating methanol to a temperature above a critical temperature of methanol to obtain a supercritical methanol. A reactive matrix is fed into the tank. The reactive matrix includes acetic acid and a material oil. A molar ratio of acetic acid to the material oil is in a range from 1:1 to 5:1. A molar ratio of methanol to the material oil is in a range from 20:1 to 90:1. A supercritical carbon dioxide is fed into the tank. A total pressure of the supercritical methanol and the supercritical carbon dioxide is higher than 10 MPa. The supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 240-500° C. and 8.1-50 MPa for 0.1-1.5 hours to obtain biodiesel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing biodiesel and, more particularly, to a method for producing biodiesel by using a supercritical fluid.

2. Description of the Related Art

Biodiesel (fatty acid methyl ester or FAME) is one of bioenergy resources for replacing fossil fuels to slow down the global warming effect.

Conventional methods for producing biodiesel, including a conventional base catalytic method using a catalyst (an alkaline substance, such as sodium hydroxide or potassium hydroxide) and a conventional supercritical alcohol production methodology, proceed with a transesterification reaction and separate free fatty acids from animal fats, plant oils, or waste oils for conversion into useful biodiesel. However, in the conventional base catalytic method, the alkaline substance is liable to reside in biodiesel in addition to generation of a large amount of glycerol as a by-product that is difficult to remove (see the reaction formula shown in FIG. 1), failing to provide a satisfactory conversion rate. In the conventional supercritical alcohol production methodology, biodiesel still has a large amount of glycerol as the by-product although catalysts are not used.

In another conventional method for producing biodiesel disclosed in Taiwan Patent Publication No. 201105789, a large amount of acetic acid is used to hydrolyze fatty acids in Jatrapha curcas L. seed oil, and methanol is then used to proceed with supercritical methanolysis, obtaining biodiesel. The conventional method includes a two-stage reaction (including a subcritical hydrolysis reaction and a supercritical methanolysis reaction) to complete production of biodiesel. The subcritical hydrolysis reaction requires a lot of acetic acid, and free fatty acid is then hydrolyzed and separated from acetic acid. The procedures are complicated while the disadvantage of a large quantity of glycerol as the by-product also exists. Thus, the conventional method can not be used to effectively produce biodiesel.

In another conventional method proposed by Zul Ilham and Shiro Saka, biodiesel is produced from supercritical dimethyl carbonate using methyl acetate as a catalyst (“A new process for catalyst-free production of biodiesel using supercritical methyl acetate”, 2009; “Dimethyl carbonate as potential reactant in non-catalytic biodiesel production by supercritical method”, 2009). Although the conventional method can effectively reduce the amount of glycerol as the by-product, formation of dimethyl carbonate by chemical engineering is costly and requires complicated operation, causing a bottleneck to production of biodiesel.

Furthermore, in the conventional methods for producing biodiesel mentioned above, plant oils preferable in maintaining the quality of biodiesel necessitate additional extraction process, increasing the costs and time for producing biodiesel.

Thus, a need exists for a novel method for producing biodiesel that mitigates and/or obviates the above disadvantages.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide a method for producing biodiesel by using inexpensive and easy-to-obtain materials.

Another objective of the present invention is to provide a method for producing biodiesel while generating a less amount of glycerol, avoiding an increase in the costs.

A further objective of the present invention is to provide a method for producing biodiesel by directly using various cooking oils or waste cooking oils as material oils to simplify the process.

The present invention fulfills the above objectives by providing a method for producing biodiesel including feeding methanol into a tank and heating methanol to a temperature above a critical temperature of methanol to obtain a supercritical methanol. A reactive matrix is fed into the tank. The reactive matrix includes acetic acid and a material oil. A molar ratio of acetic acid to the material oil is in a range from 1:1 to 5:1. A molar ratio of methanol to the material oil is in a range from 20:1 to 90:1. A supercritical carbon dioxide is fed into the tank. A total pressure of the supercritical methanol and the supercritical carbon dioxide is higher than 10 MPa. The supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 240-500° C. and 8.1-50 MPa for 0.1-1.5 hours to obtain biodiesel.

Preferably, the molar ratio of methanol to the material oil is 60:1.

Preferably, the molar ratio of acetic acid to the material oil is 3:1.

Preferably, a volumetric ratio of methanol to the material oil is 2.5:1.

Preferably, a volumetric ratio of acetic acid to the material oil is 1:6.4.

Preferably, methanol is heated to a temperature in a range of 240-500° C. to obtain the supercritical methanol.

Preferably, the material oil is selected from the group consisting of plant oils, animal oils, waste cooking oils, and oils or fats containing glycerol ester and fatty acids and combinations thereof.

Preferably, the material oil is soybean oil.

Preferably, supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 280° C. and 20 MPa.

Preferably, supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank for 90 minutes.

By using the method for producing biodiesel according to the present invention, the amount of glycerol as the by-product can be effectively reduced while saving the costs and operation time as well as recycling and reusing waste cooking oils.

The present invention will become clearer in light of the following detailed description of illustrative embodiments of this invention described in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The illustrative embodiments may best be described by reference to the accompanying drawings where:

FIG. 1 shows a reaction formula of producing glycerol of a by-product in the conventional method.

FIG. 2 shows a reaction formula in a method for producing biodiesel according to the present invention.

FIG. 3 shows another reaction formula in the method for producing biodiesel according to the present invention.

FIG. 4 shows the other reaction formula in the method for producing biodiesel according to the present invention.

FIG. 5 shows a schematic diagram of a device for producing biodiesel according to the present invention.

All figures are drawn for ease of explanation of the basic teachings of the present invention only; the extensions of the figures with respect to number, position, relationship, and dimensions of the parts to form the preferred embodiments will be explained or will be within the skill of the art after the following teachings of the present invention have been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following teachings of the present invention have been read and understood.

DETAILED DESCRIPTION OF THE INVENTION

A method for producing biodiesel according to the present invention includes feeding methanol into a tank; heating methanol to a temperature above a critical temperature of methanol to obtain a supercritical methanol; feeding a reactive matrix containing acetic acid and a material oil with a molar ratio of acetic acid to the material oil in a range from 1:1 to 5:1 into the tank, wherein a molar ratio of methanol to the material oil is in a range from 20:1 to 90:1; and adding a supercritical carbon dioxide into the tank, with a total pressure of the supercritical methanol and the supercritical carbon dioxide being higher than 10 MPa. The supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 240-500° C. and 8.1-50 MPa for 0.1-1.5 hours to obtain biodiesel.

Specifically, the method for producing biodiesel according to the present invention uses a suitable amount of acetic acid as a reactive matrix for transesterification reaction. Acetic acid can catalyze hydrolysis of fatty acids and proceed with a reaction shown in FIGS. 2-4. Therefore acetic acid completely reacts for subsequent reaction with methanol in the method for producing biodiesel without the need of separation steps. Preferably, the molar ratio of acetic acid to the material oil is 3:1, and a volumetric ratio of acetic acid to the material oil is 1:6.4. Preferably, the molar ratio of methanol to the material oil is 60:1, and a volumetric ratio of methanol to the material oil is 2.5:1. Thus, acetic acid completely reacts and reduces the yield of glycerol as a by-product.

With reference to FIGS. 1-3, acetic acid assists in separation of fatty acids from the material oil, produces free fatty acids and the free fatty acids react with methanol to obtain glycerol as the by-product (see FIG. 1). Nevertheless, the acetic acid used in the method according to the present invention has a suitable amount to react with glycerol as the by-product to form glycerol ester, reducing the amount of glycerol, as shown in FIG. 4. Furthermore, acetic acid reacts with methanol to generate methyl acetate and dimethyl carbonate to increase the conversion rate of biodiesel and to reduce the amount of glycerol as the by-product, as shown in FIG. 2.

In an example, methanol is fed by a pump into the tank. Preferably, methanol is 99% methanol, and acetic acid is 99% glacial acetic acid to completely react with methanol in a manner mentioned in the above paragraph without the need of additional processes for separation of unreacted acetic acid. The reaction time was about 0.1-1.5 hours, depending upon the size of a reaction system (for example, the tank mentioned above). The larger the reaction system is, the longer the reaction time is. As an example, the reaction time is about 15 minutes for a reaction system of 6 ml or about 1.5 hours for a reaction system of 150 ml.

Furthermore, methanol is heated to a temperature above its critical temperature, preferably in range of 240-500° C., more preferably 280° C., to obtain supercritical methanol for assisting in the reaction (the reaction shown in FIG. 1) and to increase the reaction rate.

The material oil is selected from the group consisting of plant oils, animal fats, waste cooking oils, and oils or fats containing glycerol ester and fatty acids and combinations thereof. In this example, the material oil is waste cooking oil to recycle the waste cooking oil and to reduce the amount of waste cooking oil.

Finally, supercritical carbon dioxide is added to increase the solubility of supercritical methanol and the reactive matrix and to stabilize the pressure in the tank. The total pressure of the supercritical carbon dioxide and supercritical methanol is higher than 10 MPa, preferably 20-50 MPa, and more preferably 20 MPa.

FIG. 5 shows an embodiment of a device for producing biodiesel according to the present invention, including two high-pressure pumps 1 and 2, a high-pressure tank 3, and a heater 4. Methanol and the supercritical carbon dioxide are fed by the high-pressure pumps 1 and 2 into the high-pressure tank 3 and heated by the heater 4. Each high-pressure pump 1, 2 has an inlet valve b 11, 12 to control a feeding rate of the supercritical carbon dioxide or methanol. The high-pressure tank 3 is connected to a temperature controlling unit “a” and a pressure controlling unit “b” for controlling temperature and pressure in the high-pressure tank 3. Furthermore, the high-pressure tank 3 is connected to an exhaust valve “v” for discharging product.

In an example, methanol of 75.93 ml is fed by the high-pressure pump 1 into the high-pressure tank 3 (having a volume of 150 ml) and heated by the heater 4 to 280° C. Methanol transforms into its supercritical state and is mixed with the reactive matrix that is fed by the high-pressure pump 1 into the high-pressure tank 3 at a highest flow rate of 10 ml/min. The reactive matrix consists of 30 ml of soybean oil and 4.68 ml of 99% glacial acetic acid (wherein a molar ratio of methanol to soybean oil is 60:1, and a molar ratio of acetic acid to soybean oil was 3:1). Then, the supercritical carbon dioxide is fed by the high-pressure pump 2 into the high-pressure tank 3 at a highest flow rate of 200 ml/min, such that the total pressure of the supercritical carbon dioxide and the supercritical methanol is 20 MPa. Reaction is carried out in the high-pressure tank 3 at 280° C. and 20 MPa for 90 minutes, generating biodiesel that is discharged via the exhaust valve “v”. Table 1 shows a result of the example according to the present invention. Referred to the reaction formula and a formula shown below, the conversion rate of biodiesel is 97.83% and an amount of glycerol as the by-product in the method according to the present invention is reduced by 30.2% in comparison with the conventional base catalytic method or the conventional supercritical alcohol production methodology. The formula used is listed below:

(molecular weight of glycerol×1)/(molecular weight of methanol×3+molecular weight of triglyceride×1)×100%=yield of glycerol

TABLE 1 glycerol as conversion rate by-product (%) of FAME (%) Conventional 10.26   96% method Method of the 3.09 97.83% present invention

In view of the foregoing, the method for producing biodiesel according to the present invention effectively reduces the amount of glycerol as the by-product. The method for producing biodiesel uses a suitable amount of acetic acid and methanol as the reactive matrix such that biodiesel can be produced in a single stage without additional steps of separating unreacted reactive matrix, saving the costs and operation time.

Furthermore, the method for producing biodiesel according to the present invention can be used in various cooking oils, even waste cooking oils. Thus, waste cooking oils can be recycled and reused while assuring the quality of biodiesel.

Thus since the invention disclosed herein may be embodied in other specific forms without departing from the spirit or general characteristics thereof, some of which forms have been indicated, the embodiments described herein are to be considered in all respects illustrative and not restrictive. The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

What is claimed is:
 1. A method for producing biodiesel comprising: feeding methanol into a tank and heating methanol to a temperature above a critical temperature of methanol to obtain a supercritical methanol; feeding a reactive matrix into the tank, with the reactive matrix including acetic acid and a material oil, with a molar ratio of acetic acid to the material oil being in a range from 1:1 to 5:1, with a molar ratio of methanol to the material oil being in a range from 20:1 to 90:1; and adding a supercritical carbon dioxide into the tank, with a total pressure of the supercritical methanol and the supercritical carbon dioxide being higher than 10 MPa, wherein the supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 240-500° C. and 8.1-50 MPa for 0.1-1.5 hours to obtain biodiesel.
 2. The method as claimed in claim 1, wherein the molar ratio of methanol to the material oil is 60:1.
 3. The method as claimed in claim 1, wherein the molar ratio of acetic acid to the material oil is 3:1.
 4. The method as claimed in claim 2, wherein a volumetric ratio of methanol to the material oil is 2.5:1.
 5. The method as claimed in claim 3, wherein a volumetric ratio of acetic acid to the material oil is 1:6.4.
 6. The method as claimed in claim 1, wherein methanol is heated to a temperature in a range of 240-500° C. to obtain the supercritical methanol.
 7. The method as claimed in claim 1, wherein the material oil is selected from the group consisting of plant oils, animal oils, waste cooking oils, and oils or fats containing glycerol ester and fatty acids and combinations thereof.
 8. The method as claimed in claim 7, wherein the material oil is soybean oil.
 9. The method as claimed in claim 1, wherein supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank at 280° C. and 20 MPa.
 10. The method as claimed in claim 1, wherein supercritical methanol, the material oil, acetic acid, and the supercritical carbon dioxide react with each other in the tank for 90 minutes. 